lloyal Society. 453 



.which it is deposited in grains intermixed with a large proportion of 

 animal matter, constituting tjie nacreous or granular structure. The 

 former class of shells, which includes most of the turbinated uni- 

 valves, may be divided into those in which the crystals are rhombic, 

 .and those in which they are prismatic. The first are composed of 

 three distinct layers, the laminae of which are disposed differently in 

 the intermediate layer from what they are in the outer and inner 

 layers. The direction of the fibres of each being nearly at right 

 angles to that of the contiguous layer, the strength of the shell is 

 rendered considerably greater than if the arrangement of the fibres 

 had been uniform in each plate. The comparative thickness of the 

 three plates varies in different shells ; but the central plate is gene- 

 rally the thickest. The outer plate is the thinnest ; and, in some 

 shells, is easily detached, in consequence of the deposition underneath 

 it of a white film of less coherent matter. It often happens, that 

 when the animal arrives at its full size, it deposits layers of shell 

 either on the lips or the columella : and in some, as the Cypraa, an 

 additional coat, which is harder, more compact, and differently co- 

 Joured from the rest of the shell, is formed by an extension of the 

 mantle, and laid on the outside of the shell ; the part where the two 

 reflected portions of the mantle meet on the back being marked by 

 what is termed the dorsal line. 



Besides these component parts of turbinated shells, there is often 

 deposited on the sides and interior part of their cavities, especially of 

 the upper whorls, a transparent calcareous concretion. In shells of 

 which the spires are elongated and acute, as in the Turritella, this 

 deposition entirely fills up the cavity of the upper whorls j thus ren- 

 dering solid the tips, which, from their small size and original thin- 

 ness, would otherwise have been very liable to be broken. In other 

 cases the animal, instead of filling up this upper cavity, suddenly 

 withdraws its body from the upper whorls, and then forms a concave 

 septum, by which the vital communication between the body and the 

 apex of the shell being cut off, this part decays as a dead shell, and 

 gradually falls to pieces. 



Shells having a prismatic crystalline structure are formed of short 

 fibres, everywhere perpendicular to the surface. The prisms are 

 mostly hexagonal. Shells of a granular structure present a more 

 uniform texture j the plates of animal matter they contain being very 

 thin, and closely compacted together. They have generally a pearly 

 or iridescent lustre, arising from this peculiar conformation. The 

 particles of disintegrated Placunce are employed by the Chinese as 

 silver in their water-colour drawings. In many shells belonging to 

 this class, as in the Oyster, the animal matter, being more abundant, 

 produces a distinctly laminated texture. 



It has been generally believed, and sometimes strenuously main- 

 tained, that molluscous animals have not the power of absorbing the 

 matter of their shells when it has once been deposited. The author 

 brings forward a large mass of evidence in proof of their frequently 

 exercising this power. In the Cone and the Olive, all the septa be- 

 tween the whorls inclosed in the body are very thin and transparent, 

 and, when compared with the corresponding portions of the outside, 



